Process for coating ferrous surfaces



United States Patent Office 3,516,875 PROCESS FOR COATING FERROUS SURFACES Werner Rausch, Stierstadt, Taunus, and Han Yong Oei, Hans-Joachim Edler, and Herbert Liebl, Frankfurt am Main, Germany, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed June 7, 1967, Ser. No. 644,069 Claims priority, application Germany, June 11, 1966, 1,521,876 Int. Cl. C231? 7/10 US. Cl. 148-6.15 Claims ABSTRACT OF THE DISCLOSURE A process for phosphatizing ferrous metal surfaces using a zinc phosphate coating solution accelerated with nitrate or nitrate-nitrite accelerators, wherein lead, in the form of a water and/or phosphoric acid soluble compound is added to the phosphatizing solution. The incorportion of the lead in these phosphatizing baths results in an acceleration of the oxidizing action of the nitrate or the nitrate-nitrite accelerators on the divalent iron in the bath, thus converting the dissolved iron in the 'bath to the trivalent state and maintaining the solution substantially free of divalent iron. Additionally, the presence of the lead in the solution increases the amount of nitrite re generated from the nitrate in the bath, thus maintaining sufficient nitrite ion concentration in the bath to keep the bath sufiiciently free of divalent iron without the necessity for replenishing nitrite in the bath. Desirably, the lead is added in an amount of at least about milligrams per liter, preferably in the form of lead nitrate, lead acetate, lead oxide, or lead carbonate.

This invention relates to a method for treating metal surfaces and more particularly relates to an improved process for forming a phosphate coating on ferrous metal surfaces.

It has long been the practice to utilize phosphate solutions to form protective and/or paint-base coatings on ferrous metal surfaces. Frequently, the coating solutions have been aqueous solutions of acid zinc phosphate, which solutions have also contained various accelerating agents, such as nitrate ions and nitrite ions, and the like. In many instances, in the operation of such coating baths, it has been found to be desirable if the processing solutions are maintained substantially free of ferrous ions, i.e., divalent iron. Typically, this is done by effecting oxidation of substantially all of the dissolved iron in the processing solution to the trivalent state.

Under some operating conditions, as for example where relatively high solution temperatures are used and the throughput of the ferrous metal surfaces to be treated is not to large, the oxidation rate of the nitrate and/or nitrite accelerators in the bath will be sufiicient to convert substantially all of the dissolved iron into the trivalent state, thus maintaining the bath substantially free of divalent iron. On some instances, it has actually been found that nitrite ions will :be regenerated in the processing solution, by the reduction of the nitrate ion, so that the oxidizing characteristics of the bath will be continually maintained. This method of operation is very desirable to achieve, inasmuch as it eliminates the necessity for the additional replenishment of the nitrite ions in the bath as it is operated. Unfortuniately, it has been found that in all cases, the oxidation rate of the nitrate and/or nitrite accelerators on the divalent ion and the regeneration of the nitrite from the nitrate is not sufficient to maintain the processing solution substantially free of divalent ion during the processing period. This has been found particularly to be true when low operating Patented June 23, 1970 temperatures in the bath are used and/ or when a relatively high throughput of ferrous metal through the bath is used.

It has been found that by adding copper salts to the phosphatizing bath the formation of nitrite ions from nitrate ions is accelerated. The copper ions, however, have a stabilizing effect on the ferronitroso complexes which are present in the bath so that the processing solution becomes enriched with nitroso gases. These gases are frequently evolved from the bath, particularly upon the introduction and withdrawal of the work pieces or during removal of sludge from the bath, thus endangering the operating personnel. Additionally, the presence of copper in the bath often leads to the formation of copper phosphate layers on the metal surfaces, often containing metallic copper, which layers are disadvantageous in many instances.

It is, therefore, an object of the present invention to provide an improved method for forming a phosphate coating on ferrous metal surfaces, using a nitrate and/or nitrite accelerated zinc phosphate solution.

A further object of the present invention is to provide an improved method for operating a nitrate and/or nitrite accelerated zinc phosphatizing solution whereby the necessity for adding nitrite to the processing bath during operation is eliminated.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a process for treating ferrous metal surfaces to provide therein -a protective and/or paint-base coating which comprises contacting ferrous metal surface to be treated with an aqueous acidic zinc phosphate solution, substantially free of divalent iron, which solution contains nitrite ions and an accelerating amount of lead, and maintaining said solution in contact with the ferrous metal surface for a period suflicient to form the desired coating. It has been found that by the addition of the lead to the above processing solutions, the solutions are maintained substantially free of divalent iron over a wide range of operating temperatures and work throughput, without the necessity of adding additional nitrite ions to replenish the solution.

7 More specifically, in the practice of the method of the present invention, the metal coating composition used is an aqueous acidic zinc phosphate solution substantially free of divalent iron, which solutions contains zinc ions, phosphate ions, nitrate ions, nitrite ions, and lead. Desirably, these components are present in the treating solution in the following amounts:

Lead, at least 20 milligrams per liter.

The lead added to the processing solutions of the present invention is desirably in the form of a water and/or phosphoric acid soluble compound. Exemplary of such compounds which may be used are lead nitrate, lead acetate, lead oxide, lead carbonate, and the like. These lead compounds may be added directly to the phosphatizing solution after it has been formulated although, desirably, the lead compounds are incorporated in the concentrate compositions used in making up and replenishing the phosphatizing solution. The amount of the lead compound added, either to the formulated phosphatizing solution or included in the concentrate compositions will be suflicient to provide an accelerating amount of lead in the phosphatizing solution. Generally, it has been found that amounts of lead of at least about 20 milligrams per liter of the phosphatizing solution are sufficient to provide the desired acceleration. Amounts of lead in excess of this, i.e., up to the maximum solubility of the lead compound in the phosphatizing solution may also be used.

It is to be appreciated that in addition to the above components, the metal treating solution of the present invention may also contain additional adjuvants, including metal ions such as nickel, cobalt, lithium, bismuth, cesium, and magnesium. Accordingly, in a preferred composition, the processing solutions of the present invention may contain the following components in the amount indicated'.

Components: Percent by weight Zinc 0.45-1.85 P 0.4-1.7 N0 0.461.9 N0 0.002-0.04 Nickel 00012-0005 Lead, 25 milligrams per liter to 150 milligrams per liter.

In preparing these processing solutions, various suitable materials may be used to supply the desired com ponents. Exemplary of suitable starting materials are zinc oxide, phosphoric acid, zinc dihydrogen phosphate, zinc nitrate, and the like. When metallic ions such as nickel ions, are included in the composition, they may conveniently be added in the form of the nitrate, phosphate, or carbonate salt, while the lead is desirably added in the form of a water and/or phosphoric acid soluble compounds as has been indicated above. The nitrite ions may conveniently be added to the solution as the alkali metal nitrite, such as sodium nitrite. It is to be appreciated, however, that in choosing the materials to formulate the processing solutions of the present invention, materials used are desirably those which will not introduce extraneous ions into the treating soltuions, or, that at least will not introduce ions into the solution which are detrimental either to the solution itself or to the coating which is produced.

In some instances, the processing solution may be formulated, initially, without nitrite and then operated so as to form the nitrite in situ in the solution as the coating bath is used. This may be done by providing a relatively high nitrate to phosphate ratio, e.g., a ratio of at least about 1.25 to 1, in the bath as originally formulated and then coating the ferrous metal surfaces using less than the normal metal loading rate. In this manner, nitrate will be developed in the bath and, thereafter, the bath may be operated at normal loading rates and will continue to operate on the nitrite side, i.e., without the buildup of divalent iron in the bath.

The processing baths of the present invention may be applied to ferrous metal surfaces, such as iron, steel, or the like by immersing the surface in the coating solution. Desirably, the coating solution is at an elevated temperature above room temperature, i.e., 20 degrees centigrade, with temperatures of at least about 40 degrees centigrade being preferred. Although the processing solution may be operated at higher temperatures, e.g., 60 to 120 degrees centigrade, the advantages obtained by the inclusion of the lead in the coating solutions are found to be most pronounced at the lower operating temperature. Thus, as has been noted heretofore, when operating at higher temperatures, there is normally a more rapid oxidation of the divalent iron to trivalent iron and regeneration of nitrite from nitrate so that the effects of the lead addition on these conditions are found to be less pronounced. Accordingly, in many instances, operating temperatures of from about 50 to 75 degrees centigrade are preferred. When using these coating temperatures, the contact time for forming the desired coating on the ferrous metal surfaces treated Will typically be within the range of about 1 to 20 minutes, with contact times within the range of about 2 to minutes being preferred.

The phosphate coatings produced by the present method are found to have excellent protective and/or paint base properties. If desired, the phosphate coating produced may be further treated, such as by the application of aqueous solutions containing hexavalent chromium ions. Where such solutions are used, they typically contain from about 0.01 to about 1 percent by weight of CrO either alone or in admixture with other acids, such as phosphoric acid, or the like. Additionally, it is frequently desirable to rinse the phosphate coated metal sur face with water before any subsequent treatment and then, after the final rinse, the surface is preferably dried in either air, in a drying oven, or the like.

In order that those skilled in the art may better understand the present invention in the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, the temperatures are in degrees centigrade and parts percent are by weight. It is to be appreciated, however, that these examples are intended merely to be illustrative of the invention and are not to be taken as a limitation thereof.

EXAMPLE 1 An aqueous phosphatizing solution was prepared containing the following components in the amounts indicated:

Components: Grams per liter Zinc 14.3 P 0 107 N0 21.3 NaNO 0.2 Nickel 0.03

The pointage of this bath was 40, the point being equal to the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a 10 milliliter sample of bath solution to the phenolphthalein end point. Steel plates, which had been degreased in perchloroethylene vapors and pickled in 15 percent hydrochloric acid at room temperature and then thoroughly rinsed in cold Water were immersed in the phosphatizing solution, which was at a temperature of 55 degrees centigrade for 10 minutes. The steel plates were phosphatized in the bath at the rate of 0.2 square meter of steel plate surface per liter of solution per hour. After one hour, the solution was found to be clear, dark brown color and gave a strongly positive reaction to the analysis for divalent iron in the bath.

EXAMPLE 2 The procedure of Example I was repeated with the exception that about 1.2 milligrams per liter of lead was added to the phosphatizing bath, as lead nitrate. After operating the bath for one hour, as in Example 1, the bath was found to be clear and dark brown in color and gave a strongly positive reaction to the analysis for divalent iron.

EXAMPLE 3 The procedure of Example 1 was repeated with the exception that 34 milligrams per liter of lead was added to the phosphatizing bath, as lead nitrate. After operation of the bath for one hour, as in Example 1, the bath was found to be milky white in color and gave a substantially negative reaction to the analysis for divalent iron.

EXAMPLE 4 The procedure of Example 1 Was repeated with the exception that milligrams per liter of lead was added to the phosphatizing bath, as lead nitrate. After operating the bath for one hour in accordance with the procedure of Example 1, it was found that the bath was milky white in color and gave a substantially negative reaction to the analysis for divalent iron.

The procedure of the above Examples 3 and 4 is repeated with the exception that the lead is added to the phosphatizing bath as lead acetate, lead oxide, and lead carbonate, rather than lead nitrate. In each instance, similar results are obtained in terms of maintaining the processing bath substantially free of divalent iron.

In the above Examples 3 and 4, the phosphate coatings produced on the steel panel are found to have excellent protective and/ or paint-base properties. In the operation of the coating baths of these examples, it was found that almost immediately upon immersing the steel panels in the solution, the solution became turbid and substantially all of the dissolved iron in the bath was transformed, during the bath operation into the difiiculty soluble trivalent iron phosphate. In contrast, in the coating baths of Examples 1 and 2, appreciable amounts of divalent iron are contained in the bath as shown by the discoloration of the bath solution to a dark brown color indicating the formation of soluble ferronitroso complexes, While the baths remained clear, indicating the lack of formation of the trivalent iron phosphate sludge in the bath.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention and it is realized that changes therein are possible and it is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A process for coating ferrous metal surfaces to provide a phosphate coating thereon which comprises contacting the surface to be coated with a coating composition comprising an aqueous acidic zinc phosphate solution, substantially free of divalent iron, which solution contains nitrite ions, and at least 20 milligrams per liter of lead, and maintaining the coating solution in contact with the metal surface for a period sufiicient to form a phosphate coating, whereby the solution is maintained substantially free of divalent iron during the treatment of the ferrous metal surfaces without the further addition of nitrite ions.

2. The method as claimed in claim 1 wherein the coating solution contains from about 0.05 to 5.0 percent by weight zinc, from about 2.0 to 10 percent by weight P04, from about 0.001 to 0.08 percent by weight of NQ and from about 0 to 10.0 percent by weight NOE.

- 3. The method of claim 2, wherein the coating composition also contains nickel ions in an amount within the range of about 0.001 to about 0.005 percent weight of the coating solution.

4. The method as claimed in claim 3 wherein the lead is to the solution in the form of a water or phosphoric acid soluble compound.

5. The method as claimed in claim 4 wherein the lead is added as lead nitrate.

References Cited UNITED STATES PATENTS 1,949,090 2/ 1934 Tanner et al.

2,329,065 9/ 1943 Lum et al. 1486.15

2,813,812 11/1957 Somers et al.

2,863,793 12/1958 Cerma.

3,166,444 1/ 1965 Ehren et al. 1486.15 X

3,261,723 7/1966 Craig 1486.l5

3,338,755 8/1967 Jenkins et al.

3,364,081 1/1968 Forsberg 1486.15

FOREIGN PATENTS 1,060,693 7/ 1959 Germany.

ALFERD L. LEAVI'IT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner 

